General polymer electrolytes are made by mixing a large amount of polymer with a small amount of lithium salt (usually it is necessary to dissolve the two with some kind of volatile solvent, and then remove the solvent). Since the amount of polymer is much greater than the amount of salt, this polymer electrolyte can be referred to as a salt in polymer electrolyte. But in 1993 Angell et al. did the opposite. They mixed a large amount of lithium salt with a small amount of polymer polyoxypropylene and polyoxyethylene to obtain a polymer electrolyte that was different from the traditional one. They call this new type of polymer electrolyte a polymer in salt electrolyte. The mechanical properties of these polymer electrolyte materials are very good and it is a strong material. Their glass transition temperature is lower than room temperature. In most cases, they have the solid characteristics of rubber, and at the same time have high lithium ion conductivity, good electrochemical stability and good compatibility with metal lithium. Therefore, this type of polymer electrolyte quickly attracted a large number of researchers and developers of polymer lithium-ion batteries.
With a short spacer group polyanionic polymer [poly (lithium oligoetherato monooxalato orthoborate), called polyMOB or P (LiOEGnB). Where n represents the repeating unit of oxyethylene] is a polymer ionic plasticizer, and LiClO4, LiTFSI and LiBF4 are used as electrolyte salts, respectively. The "polymer in salt" ion conductivity behavior is observed in a wide range of salt concentration. Although all these lithium salts can form rubbery solids at high salt content, only ionic rubber using LiClO4 can provide high electrical conductivity [when n=14, the single ion conductivity at 25℃ reaches 10-5S/cm, and the electrochemical stability window exceeds 4.5V (relative to Li+/Li)], and the addition of lithium salts can reduce the glass transition temperature of the polymer. Because the salt concentration in ionic rubber is very high, the efficient transport of ions in this type of electrolyte must be related to the high concentration of ions in it. Therefore, in order to provide ionic carriers, on the one hand, the dissociation energy of the lithium salt used must be low enough, and at the same time, no or only a small amount of solvent should be used in this type of electrolyte. The separated ion clusters regroup together to form an infinite ion cluster, which promotes the rapid transport of ions in the entire electrolyte.
Heat the dried PC/PAN/LiTFSI mixture with appropriate ratio at 150°C for 8~10h, and part of the electrolyte prepared in this way is enclosed between two glass inner sheets for Raman measurement, and the other part is installed in a "stainless steel/polymer electrolyte/stainless steel" sample cell for impedance spectroscopy measurement from low temperature to high temperature. The time interval between each two temperature points is 40~60min to ensure the temperature balance between the sample cell and the dry silica gel. Bury the sample cell in dry silica gel to prevent the sample from absorbing water. The special design of the sample cell allows the excess electrolyte caused by the volume expansion of the electrolyte to flow out along the reserved small holes due to the increase in temperature, which ensures that the area and thickness of the electrolyte sample will not change due to the increase in temperature, which will affect the impedance measurement results.
Figure 1 shows the effect of salt concentration on the conductivity of polymer electrolytes. Since the polymer does not dissolve before all PC molecules associate with lithium ions, the concentration of salt used is higher than that of the normal (optimal) practical polymer electrolyte. Therefore, the conductivity of the polymer electrolyte is not high. It can be seen that the conductivity of the electrolyte decreases rapidly as the salt content increases. This result is consistent with the conduction behavior of most liquid or polymer electrolytes. In those electrolytes, after a maximum conductivity value, the conductivity of the electrolyte decreases as the salt concentration further increases. It should be pointed out that when the molar ratio of PC:PAN:LiTFSI exceeds 1:1:4, the conductivity of the electrolyte in turn increases with the increase in salt content. In order to understand this transition in the conduction behavior of polymer electrolytes, the Raman spectra of these polymer electrolytes can be measured. Figure 2 shows the Raman spectrum of PAN's C-N stretching vibration peak (2245cm-1) as a function of salt content. As the salt content increases, a new peak splits at 2270cm-1, indicating that the association between lithium ions and the cyano group C≡N of PAN has occurred. When the salt content exceeds 1:1:3, the 2245cm-1 peak disappears, and the 2270cm-1 peak is dominant.
Figure 1 - Arrhennius diagram of different molar ratios of polymer charge and PC:PAN:LiTFSI
Figure 2 - Raman spectrum of the C≡N stretching vibration peak (2245cm-1) in PAN as a function of salt content
When the salt concentration continues to increase, a new peak from the 2270cm-1 peak can be observed at 2280cm-1. When the salt concentration reaches 1:1:5, this new peak becomes very obvious. The appearance of this new peak is directly related to the change in polymer conductance behavior at high salt concentrations.
Figures 3 and 4 are the Raman spectra of the CF3 symmetric stretching vibration mode (at 742 cm-1 in the PC/LiTFSI dilute solution) and the S-N stretching vibration mode (at 790 cm-1 in the dilute solution) in the anionic TFSI-, respectively. In the 1.53mol/L PC/LiTFSI solution, a new peak at 800cm-1 split from the 790cm-1 peak, indicating that an ion pair is formed in the solution. It can be seen that as the salt content increases, the positions of the S-N stretching vibration and CF3 symmetric stretching vibration peaks move to the high wave number end. When the salt concentration reaches 1:1:6, the peak becomes similar to that of pure LiTFSI. These spectral changes indicate that strong ion association has occurred in the electrolyte, although no new peaks appear from the outline of the polymer electrolyte. Therefore, the peak at 2280 cm-1 can be considered to be caused by the interaction of the cyano group of PAN with ion aggregates and ion clusters Lim+TFSIn- (m>n). The interaction between a single ion and a cyano group will form the usual Li+...N≡C-R association, while the interaction of an ion cluster with the cyano group of PAN will form a Lim+TFSI+N≡C-R association. In addition, there will be free ion groups or ion clusters Lim+TFSI+ that are not associated with the polymer. On the other hand, due to the plasticizing effect of the solvent PC and the salt itself, the ion cluster Lim+TFSIn- will move with the polymer segment like a single lithium ion. Since both types of ions contribute to the ionic conduction in the electrolyte, the ionic conductivity of the polymer electrolyte increases as the salt content increases.
Figure 3 - Raman spectrum of CF3 symmetric stretching vibration mode in anionic TFSI-
Figure 4 - Raman spectrum of S-N stretching vibration mode in anionic TFSI-
Only when the ionic clusters are connected into an infinitely long chain of ionic clusters, the polymer in salt transition will occur. Before the formation of an infinite chain of ionic clusters, this transition in conduction behavior has already occurred. For example, conduction occurs when PAN: salt=10:1. If an ion cluster chain must be formed first and connected into an infinitely long percolation channel for the rapid transport of cations in the entire electrolyte, then ion clusters must be formed first. Then a sufficient number of ion clusters are connected to each other to form a percolation channel for this transformation to occur. Due to the presence of solvent in the polymer electrolyte, ion clusters were not observed until PC:PAN:LiTFSI=1:1:3. This shows that the transition of conductance behavior occurs before the formation of a large number of ion clusters. Therefore, we propose that in addition to helping the dissociation of the lithium salt, another important role played by the polymer is to realize the transformation of the conductance behavior.
In polymer electrolytes, the relationship between polymer and ion transport is as follows: in a polymer electrolyte with a low salt concentration (compared to the salt concentration corresponding to the formation of the ion cluster percolation channel), the cations of the dissolved salt associate with the polymer and migrate with the movement of the polymer segment. This corresponds to the 2245cm-1 peak split in the Raman spectrum and the 2270cm-1 component appears. When more lithium salt is dissolved in the polymer, the ion associations grow up into ion clusters and interact with the polymer. This corresponds to the 2270cm-1 peak in the vibrational spectrum to further split the mountain 2280cm-1 peak, but this time the formation of effective percolating clusters, and the movement of lithium ions and polymer segments associated with Lim+TFSIn- clusters is still the main contributor to the conductivity of the polymer electrolyte. When the salt concentration becomes very high, some ion clusters begin to form, but they are still not enough to connect to each other to form an effective percolation channel. In this case, the polymer segments associated with ions or ion clusters become instantaneous bridges connecting adjacent ions or ion clusters. Since the movement of the polymer chain segment does not necessarily coincide with the jumping of ions or ion clusters, the conductivity of the polymer electrolyte is still not high at this time. When the concentration of the salt becomes extremely high (close to the solubility limit of the lithium salt in the polymer and solvent), an effective percolation channel is formed and has the greatest contribution to the conductivity of the polymer electrolyte.
Polymers have different effects on conductivity in different salt concentration ranges. In the commonly used gel polymer electrolyte, the lithium salt is well dissolved by the solvent, and the role of the polymer is to act as a skeleton of the salt solution to provide the electrolyte with sufficient elasticity and mechanical strength. At this time, the carriers move in a liquid-like environment. In this case, the salt concentration should not be too high and the association of ions with the polymer body should be avoided. Because the movement of polymer segments is much slower than the movement of free ions in the liquid state. This is the mechanism of movement of ions in the usual salt in polymer. But when the salt concentration is very high, the polymer becomes the main host of carriers. Due to the high salt concentration, all solvent molecules (if any) are associated with ions, and these lithium ions cannot move with the solvent molecules. Therefore, lithium ions must rely on the movement of polymer segments to achieve migration. In this case, the dissociation of the salt and the association of the ion with the polymer become very important for the polymer to conduct electricity. Before forming a percolation channel through the electrolyte, ion transport, which is closely related to the movement of polymer segments, is the main form of polymer conduction. The types of ions that associate with the polymer are also very important, because they determine the effectiveness of the segment motion to contribute to the conductivity. With the further increase of salt concentration and the formation of effective percolation channels, the contribution of ion cluster transport to the total conductivity gradually dominates.